1. Field of the Invention
The present invention relates to an engine operating apparatus for an internal combustion engine supplied with a very lean air-fuel mixture near the misfire limit.
2. Description of the Prior Art
It is known in the prior art that a high combustion efficiency and fuel consumption efficiency are obtained with a lean air-fuel mixture of a ratio higher than the theoretical air-fuel ratio, i.e., higher than about 14.5. It is also known that the emission of nitrogen oxides from the engine may be reduced in the lean air-fuel mixture region.
An engine supplied with such a lean air-fuel mixture, however, must have an ignition timing highly responsive to changes in the air-fuel ratio. For example, if the air-fuel ratio changes 1.0, the ignition timing must be changed 3 to 6 degrees. This means that both the air-fuel ratio and ignition timing must be precisely controlled.
In a heretofore-provided lean burn internal combustion engine, air-fuel ratio is maintained by arranging a so-called "lean sensor" in the flow of the exhaust gas to detect the concentration of an exhaust gas component; using signals from the sensor to determine the air-fuel ratio during engine operation; and feed-back controlling the amount of fuel to be injected.
This type of engine control apparatus, however, suffers from the drawback of a large time lag in control due to the delayed transmission of fuel from the fuel injection valves and the attachment of liquid fuel onto the inner surface of the intake pipe. The relationship between the air-fuel ratio and ignition timing is that determined under the engine operating conditions, e.g., combination of the engine rotational speed and the intake air pressure, of that particular moment. Therefore, if the air-fuel ratio is off from the target value, the engine will operate under an ignition timing off from the required value, i.e., an advanced or delayed timing, resulting in poorer drivability, increased emission, and decreased fuel consumption efficiency.